The “medium energy” is represented by the In-In data obtained by the NA60 collaboration.
Note that this is the only medium size collision system included in the systematics.
For this case \(R\)  was computed from the ratios published in [1]} where the continuum yield, normalized to a so-called cocktail \(\rho \)  is presented for 12 centrality bins.
To include these data to our systematics, the continuum yield was assumed to be constantindeed the continuum yield measured in the NA60 acceptance appears nearly constant due to the particular acceptance filter of the spectrometer
and accounted for in the \(200-600\)   G\(e\) V/\(c^{2}\)  invariant mass region (for details see e.g. [2]}).
The ratios originally presented refer to the excess yield (in acceptance) without \(\rho \)  peak in the \(200<M_{\mu ^+\mu ^-}<1000\)   M\(e\) V/\(c^{2}\)  relative to the cocktail \(\rho \)  yield.
The latter is calculated from the measured \(\omega \)  yield assuming that the \(\rho \)  signal is as strong as the \(\omega \)  signal.
The respective cocktail yield has been obtained from the centrality integrated spectrum accounting for \(\eta \)  and \(\omega \) -Dalitz decays in the \(200-600\)   G\(e\) V/\(c^{2}\)  mass range.
The cocktail was assumed to scale linearly with the charged particle multiplicity as reported in [1]}.
The data points (denoted by diamond symbols) are plotted as function of the charged particle multiplicity density.
The data collection from SPS is complemented with the data obtained by the CERES collaboration for Pb-Au collisions also at \(\sqrt{s_{\mathrm {NN}}}\, = 158 \, \mathrm {G}e\mathrm {V}\)  [4]}.
The ratios \(R\)  have been derived as signal over cocktail yield integrated from 200 to 600  M\(e\) V/\(c^{2}\)  and are shown (squares) for four centrality classes spanning the 40% most central collisions according to the trigger conditions used in the experiment.
